The invention concerns a mixer in particular for viscous elastic materials having a processing channel with a closeable outlet opening at its end and along which the materials can be transported by means of a transport device while being simultaneously mixed and with a feedback channel which branches off proximate to the outlet end of the processing channel and connecting back into this channel at an upstream location, wherein the material mixture can be circulated through the processing channel and the feedback channel when the outlet opening is closed.
A mixer of this kind can also serve as an extruder if the outlet opening is properly configured in a conventional manner and the expression "mixer" explicitly includes same.
Extruders generally have an elongated processing channel in which one or more worm conveyors are disposed. Differing amounts and kinds of mixing components, in particular high molecular viscous elastic materials such as polymers or elastomers, are introduced through a filler chute into the processing channel. The mixing components are subjected, within the processing channel and by means of the worm conveyors, to mechanical deformation forces and are thereby reformed and kneaded into a viscous elastic mass which is as homogeneous as possible. The processing characteristics and the viscosity of high molecular materials such as polymer melts or elastomers is determined by a combination of its viscous and elastic properties, wherein, among other things, the kind and the fractions of mixing components as well as the quality of the degree of distribution of all components have a substantial influence. The mixing components can be polymers having differing molecular structures or can include powdered filling materials having different particle sizes and differing particle size distributions. During mixing or kneading of the differing components, it is difficult or impossible to predict how the viscous or elastic properties of the mixture will change during the mixing or kneading process. It is also not possible to e.g. reliably predict the extent to which the processing characteristics of the mixture are changed by the mixing procedure in consequence of degradation and which useful properties the mixture will finally have. It is therefore of interest to be able to track both the viscosity as well as the elasticity of a mixture during the mixing process itself in dependence on the processing conditions and the mixing time.
These rheological quantities (primarily the viscosity) can be determined in conventional mixers by e.g. recording, for a constant rate of revolution of the worm conveyor, the time dependence of the resistance which the mass in the mixing chamber exercises on the screws by measuring the required power (torque) of the drive motor. However, since laminar layered flow does not establish itself in the mixing chamber, as is required for absolute viscosity measurements in rheometers, the viscosity values determined by the torque of the drive device have relative character only, in particularly since the partially highly complex flow geometry in the mixer and the remaining mixing parameters can have a substantial influence on the magnitude of this quantity.
Up to this point in time, absolute rheological measurements during the mixing procedure have not been carried out to a satisfactory extent. In particular, the changing material properties during mixing (compounding) are thereby of interest. A good mixing of the components requires mixing or whirling same as long as possible in a turbulent fashion. This can be achieved by transporting the components along a relatively long processing channel or by using conventional so-called feedback mixers or mixing extruders with which the outlet opening on the end of the processing channel can be closed to feed the material back via a feedback channel branching-off proximate the outlet end of the processing channel and introduced one more into the processing channel at an upstream location. The circulation of the material mixture through the processing channel and the feedback channel thereby achieved can effect a relatively long and thereby more intense mixing process despite a relatively short construction for the mixer or the extruder.
In order to determine the rheological quantities of the mixture, conventional mixers and extruders normally have a downstream rheometer device or branch-off a portion of the instantaneously produced mixture and introduce same into a rheometer and, if appropriate, feed it back into the main flow. In the first case, a determination of the rheological quantity is only possible at the end of the mixing procedure or by interrupting same. When a sample amount is branched off, the amount of time between the branching-off of the sample amount and the actual measurement must be kept as short as possible. This requires disposing special transport pumps in the branched channel for introducing the sample amount to the measuring device, leading to a complicated and expensive construction.
It is the underlying purpose of the invention to create a mixter of the above mentioned kind with which the rheological quantities of the mixture can be reliably determined during the mixing process.